The invention relates to optical devices, and more particularly to an elastomeric, transparent element having a diffracting surface for use as a near-field, contact-mode phase mask for photolithography, a sensor of physical force, and a visual display device. The invention also relates generally to the optical patterning of surfaces via photolithography, and more particularly to photolithography in which a surface of photoresist is contoured and alters light in a manner such that the photoresist is developed according to a pattern without the use of an auxiliary mask.
Differences in phase of electromagnetic radiation can be produced by varying the path that light follows, either by passage of light through media of differing refractive index, or reflection from a corrugated surface and by interaction of light with diffraction gratings. Phase differences of light have been exploited for a variety of uses including sensors, apparatus for photolithography, and optical displays. A brief description of several examples of such uses follows.
Sensors and modulators based on changes in optical properties corresponding to changes in electric, magnetic, and acoustic fields are known. Micron-scale modulators that involve mechanical deformation of suspended beams, mirrors, and Fabry-Perot cavities are under development for applications in fiber optic communications and displays. See, for example, Leeson, et al., xe2x80x9cDesign and Fabrication of Planar, Resonant, Franz-Keldysh Optical Modulatorsxe2x80x9d, Electron. Lett ., 24, 1546-1547 (1988); Solgaard, et al., xe2x80x9cDeformable Grating Optical Modulatorxe2x80x9d, Optics Letters , 17 (9), 688-690 (1992); Feather, et al., xe2x80x9cMicromirrors and Digital Processingxe2x80x9d, Photonics Spectra , 118-124 (May, 1995).
Yamamoto, et al., in xe2x80x9cDirect Measurement of Piezoelectric Strain Using a Diffraction Gratingxe2x80x9d, J. Am. Cer. Soc ., 70, 8, 557-561 (August, 1987), describe application of a large DC electric field to a sample to induce strain in the sample in a direction parallel to a surface having a grating relief structure, and determination of the strain by detection of a change in wavelength of light in the diffraction pattern produced by the grating.
Boone, P. M., in an article entitled xe2x80x9cA Method for Directly Determining Surface Strain Fields Using Diffraction Gratingsxe2x80x9d, Experimental Mechanics , 481-489 (November, 1971), describes determination of the strain distribution at the surface of a solid body to which stress is applied by analyzing diffraction phenomena of a grating applied to the body.
Sciammarella, et al., in xe2x80x9cTwo New Optical Techniques to Measure Strainxe2x80x9d, Experimental Mechanics , 311-316 (August, 1974), describe determination of strain in an article to which stress is applied by engraving a grating into a surface of the article and determining a change in angle of a particular diffracted beam with the grating surface corresponding to a change in pitch of the grating resulting from strain. See also Olivaries-Perez, et al., xe2x80x9cDynamic Holographic Gratings With Photoresistxe2x80x9d, Applied Optics , 34 (25), 5577 (1995).
Post, et al., in xe2x80x9cHigh-Sensitivity Moirxc3xa9 Interferometryxe2x80x94A Simplified Approachxe2x80x9d, Experimental Mechanics , 100-104 (March, 1981) report a Moirxc3xa9 Interferometry arrangement in which a collimated beam is incident upon a phase-type reflection grating on a specimen, the specimen is deformed, and determination is made of deformation parallel to the grating surface corresponding to a shift in wavelength of light diffracted at the grating.
Photolithographic techniques have exploited phase-shifts of light produced by masks. Toh, et al., in xe2x80x9cChromeless Phase-Shifted Masks: A New Approach to Phase-Shifting Masksxe2x80x9d SPIE volume 1496, Tenth Annual Symposium On Microlithography, 27-53 (1990); and Tanaka, et al., in xe2x80x9cA Novel Optical Lithography Technique Using the Phase-Shifter Fringexe2x80x9d, Japanese Journal of Applied Physics , 30 (5), 1131-1136 (1991) report a phase-edge photolithography method in which a transparent mask induces abrupt changes of the phase of light used for exposure of photoresist, causing optical attenuation at those phase-shifted locations. Very small features can be created in photoresist according to the technique, but an arrangement of imaging optics must be positioned between the phase mask and the photoresist. The requirement of optics in projection onto the photoresist limits the area of photoresist that can be patterned in a single exposure, and the technique requires precise positioning of the resist at the image plane.
The techniques discussed above involving diffraction and phase-shifting of light for a variety of purposes all involve relatively complicated and expensive optics and/or involve measurement of a change in wavelength of light associated with a shift in a fringe pattern of a diffraction grating, which itself can require relatively complicated optics for detection.
Photolithography is a procedure in which a material called photoresist is placed on a surface and exposed to electromagnetic radiation in a pattern such that certain portions of the photoresist are exposed to the radiation and certain portions are not exposed. Depending upon the particular type of photoresist used, the exposed portions or the unexposed portions are removed chemically, which exposes the surface in a pattern identical to or complementary to the pattern of electromagnetic radiation. Exposed portions of the underlying surface then can be etched away, or portions can be plated on the underlying surface, in a pattern dictated by the pattern of photoresist. This technique has been widely used in fabrication of many small-scale devices, especially microelectronic devices.
To expose photoresist to electromagnetic radiation in a specific pattern, two general techniques have been used. One has involved xe2x80x9cwritingxe2x80x9d into the photoresist with a beam of electromagnetic radiation such as a laser beam or an electron beam. Another widely-used technique involves exposing the photoresist to electromagnetic radiation through a mask. Amplitude masks and phase-shifting masks are known for use in this technique.
Although photolithography is a well-developed field, most techniques require relatively complicated and expensive apparatus. Accordingly, there is a general need to provide simplified, inexpensive, photolithographic techniques and devices for optical displays, sensors, and photolithographic masks that are relatively simple and inexpensive to fabricate and use.
The present invention provides a contact phase mask for use in photolithography. The phase mask can be used in combination with a surface, such as a surface of photoresist, to which phase-shifted radiation advantageously is applied, can include a surface conformable to the surface that receives the radiation. The mask has a first portion, having a first refractive index, positionable against a surface to receive radiation and allows for a medium having a second, different refractive index to reside adjacent the first portion, and also adjacent the surface that receives phase-shifted radiation. The phase mask can be used in combination with a system including a stage for positioning a sample including a layer of photoresist. The system is free of optical elements positionable between the mask and the photoresist.
The present invention also provides techniques involving photoreactions at surfaces, articles for use in surface photoreactive procedures, and methods of making these articles.
According to one aspect, the invention provides a method for exposing a surface to electromagnetic radiation through a phase mask. The method involves placing a surface of a phase mask in contact with a surface of an article to be exposed, and exposing the surface to electromagnetic radiation through the phase mask. The phase mask can include a contoured surface.
In one embodiment, the invention provides a method of using a phase-shifting article. The method is used in conjunction with a surface of an article to be exposed to electromagnetic radiation, and involves establishing a minimum in intensity of electromagnetic radiation at a predetermined area of the surface by directing the radiation at the predetermined area while contacting a first portion of the surface, which terminates at the predetermined area, with a phase-shifting article. The phase-shifting article is transparent or semi-transparent to the radiation and shifts the phase of the radiation. The phase-shifting article has a first refractive index, and the method involves allowing a second portion of the surface that bounds the first portion at the predetermined area to remain free of the phase-shifting article. A refractive index boundary is thereby established at the boundary of the first and second portions. This refractive index boundary creates a phase boundary in the electromagnetic radiation striking the surface at the predetermined area when electromagnetic radiation is shifted by the phase-shifting article at the first portion so as to be completely or partially out-of-phase with electromagnetic radiation striking the second portion. The second portion of the surface that remains free of contact with the phase-shifting article can remain free of contact with any article (exposed to the environment in which the technique is used) or can be contacted with a material having a refractive index different from the first refractive index.
According to one embodiment, the phase-shifting article has a contoured surface including a plurality of alternating indentations and protrusions. In this embodiment, the method involves placing outward-facing surfaces of the protrusions in contact with first portions of the surface to be exposed, while allowing the indentations to be positioned in alignment with intervening, contiguous, second portions of the surface that thereby remain free of contact with the phase-shifting article. Thus, the second portions of the surface remain exposed to the atmosphere in which the technique is carried out. Alternatively, the second portions can be contacted with a transparent material having an index of refraction different from that of the index of refraction of the protrusions. The method, according to this embodiment, involves exposing the surface to the electromagnetic radiation through the phase-shifting article thereby creating a phase boundary in the electromagnetic radiation striking the surface at a plurality of predetermined areas. In this manner, minima in intensity of electromagnetic radiation are established at each of the plurality of predetermined areas.
The phase-shifting article can be created at the surface of interest to which it is desirable to direct phase-shifted radiation. This can involve providing a surface to be irradiated and forming, on that surface, a phase mask, or forming a precursor of a phase mask followed by forming the phase mask. A film of photoresist can be applied to the surface of interest and a pattern formed in the film of photoresist by conventional photolithography, or a pattern of a polymeric, ceramic, metallic, or molecular species formed on the surface of interest to serve as the phase-shifting article. The species formed at the surface is selected to be transparent to the electromagnetic radiation to the extent that an effective phase-shifting patterning can be achieved. The species can be formed at the surface by a variety of methods. One method involves microcontact printing or micromolding.
The invention also provides a method for altering selected regions of a film of photoresist while leaving remaining regions relatively unaltered. The method involves contacting a first portion of a surface of photoresist with a transparent article while leaving a second, adjacent portion of the surface free of contact with the article thereby establishing a refractive boundary at the boundary of the first and second portions. Then, electromagnetic radiation is directed at the first and second portions through the transparent article. The transparent article establishes a first phase of the electromagnetic radiation that strikes the first portion and a second phase that strikes the second portion, the second phase being out-of-phase with the first phase. This establishes a phase boundary in the electromagnetic radiation at the refractive boundary. Electromagnetic radiation is applied to the first and second portions at an intensity and for a period of time sufficient to establish a difference in physical characteristic between the photoresist at a particular area that includes the boundary of the first and second portions and the photoresist in other areas. This difference in physical characteristic can involve altering the physical characteristic at the particular area, or altering the physical characteristic at other areas. Typically, the physical characteristic of the photoresist at areas other than the particular areas that include the boundaries of the first and second portions is altered, and after this process the photoresist at those areas is removed, leaving intact the photoresist at the areas including the boundaries, in a pattern.
The invention also provides a method of creating very small features in a film of photoresist. The method involves directing incoherent electromagnetic radiation including a component of wavelength greater than 365 nm at first and second, contiguous portions of a film of photoresist. The electromagnetic radiation is altered at the surface of the photoresist to establish a first intensity of electromagnetic radiation striking the first portion of photoresist and a second intensity of electromagnetic radiation striking the second photoresist portion. Then, the second portion of the film of photoresist is removed from the first portion, leaving the first portion which defines a photoresist structure having a lateral dimension of less than 200 nm.
In each of the described embodiments of the invention, the surface that is exposed to electromagnetic radiation and thereby altered can be nonplanar, and the pattern created at a planar or nonplanar surface via techniques of the invention can involve curved lines.
The present invention also involves etching and plating techniques making use of the described photoresist patterning techniques. In one set of embodiments photoresist is patterned on a substrate and a species is plated in remaining, uncovered regions. In another set of embodiments photoresist is patterned on a surface and uncovered, remaining portions are etched.
The present invention also provides a diffraction grating having a diffracting surface including a plurality of indentations, in which the depth of at least one indentation is adjustable.
Also provided in accordance with the invention is a transparent diffraction grating in which the depth of at least one indentation of the grating""s diffracting surface is adjustable, and a modulator constructed and arranged to change the depth of at least one indentation of the diffracting surface while allowing the depth of at least another indentation to remain unchanged. The modulator can have a surface that addresses the diffracting surface and imparts a force on the diffracting surface in a direction perpendicular to the surface, thereby compressing one or more protrusions and changing the depth of one or more indentations. The modulator can contact a first region of the grating surface and apply a force to the first portion thereby changing the depth of indentations within that first portion. A change in optical phase at the first portion is produced. A second portion of the diffracting surface that is not addressed by the modulator retains an optical phase different from the phase that is changed by the modulator.
According to another aspect, the invention provides a method that involves changing the depth of at least one indentation in a diffracting surface of a diffraction grating. The method involves diffracting light at the diffracting surface, the surface including a plurality of indentations each having a depth. A first diffraction pattern is thereby created. The depth of at least one indentation is changed, thereby creating a second diffraction pattern that is distinguishable from the first diffraction pattern. The method can involve creating a display and, where a particular region of a diffracting surface involving a plurality of indentations is desirably altered (as when the invention finds use as a display), the technique involves establishing a first optical phase at a diffraction grating surface and altering the first optical phase at a selected portion of the grating surface to create a second optical phase. The first optical phase is retained at at least one portion of the grating surface other than the selected portion.
According to another aspect, the invention provides a sensor that makes use of the diffraction of electromagnetic radiation. In one aspect, the invention provides a system that acts as a sensor. The system includes a surface that diffracts electromagnetic radiation and a detector in combination with the surface, constructed and arranged, and positionable, to detect a change in intensity of electromagnetic radiation emanating from the diffracting surface. The change in intensity is measured at an angle, relative to the surface, that remains constant.
According to another aspect the invention provides a method of determining strain in an article to which stress is applied. The method involves subjecting the article to a first stress (or leaving the article free of stress) and establishing a diffraction pattern having a beam of first intensity at the article. This diffraction pattern can be established by passing electromagnetic radiation through a surface of the article. The article then is subjected to a second stress (which can involve removing stress from the article) thereby changing the first intensity of the beam to a second intensity. The second intensity is distinguished from the first intensity, and this distinction can be quantified so as to quantify the difference between the first and second stresses. The application of stress to the article can be due to a change in temperature of the article. In one embodiment, the method involves detecting a change in a diffraction pattern, established by a diffraction grating that is in thermal communication with an article, corresponding to a change in temperature of the article. The change in temperature can be due to application of electromagnetic radiation to the article. In that embodiment, the method involves allowing electromagnetic radiation to interact with the article to change the temperature of the article, and determining the change in temperature corresponding to change in diffraction pattern.
In another embodiment, the invention provides a method of detecting electromagnetic radiation. The method involves providing an article that is absorptive of electromagnetic radiation and that is in thermal communication with a diffraction grating having a diffracting surface. A first diffraction pattern of electromagnetic radiation is established at the diffracting surface when the article is at a first temperature. Electromagnetic radiation then is directed at the article at an intensity and for a period of time sufficient to change the temperature of the article from the first temperature to a second temperature thereby altering the first diffraction pattern to a second diffraction pattern. The second diffraction pattern then is distinguished from the first diffraction pattern.
Another method of the invention involves exposing a photoreactive, contoured surface to essentially uniform electromagnetic radiation and allowing contours in the surface to alter the electromagnetic radiation. This promotes occurrence of a photoreaction at a first portion of the surface to a greater extent relative to a second portion of the surface.
Another method involves exposing a photoreactive surface of essentially even chemical composition to essentially uniform electromagnetic radiation. A photoreaction is then allowed to take place, promoted by the electromagnetic radiation. The photoreaction takes place at a first portion of the surface to a greater extent relative to a second portion of the surface.
Another method of the invention involves exposing a photoreactive surface to electromagnetic radiation through a mask. The masked electromagnetic radiation impinges upon a surface, and contours in the surface alter the electromagnetic radiation thereby promoting occurrence of a photoreaction at a first portion of the surface to a greater extent relative to a second portion of the surface.
In another aspect the invention provides an article for photoreaction. One article includes a photoreactive surface that has contours in it, constructed and arranged to alter electromagnetic radiation directed at the surface to promote occurrence of a photoreaction. The photoreaction occurs at a first portion of the surface to a greater extent relative to a second portion of the surface.
In another aspect the invention provides a method of making a photoreactive surface. The method involves constructing and arranging contours in a photoreactive surface that alter electromagnetic radiation directed at the surface to promote occurrence of a photoreaction at a first portion of the surface to a greater extent relative to a second portion of the surface.